Vapor compression systems (VCS), e.g., as air conditioners or heat pumps, are multivariable systems whose primary objective is to move a quantity of heat from, e.g., a low temperature environment to a high temperature environment in order to regulate the temperature of an indoor space. A controller for the VCS selects control values such that the measured room temperature is driven to the setpoint room temperature, and as a result, the cooling or heating energy provided by the VCS is indirectly matched to the heating or cooling load, referred herein as a thermal load.
Various conventional methods for controlling the VCS determine the required control inputs to regulate the temperatures without measuring the thermal load. This is because the VCS is installed in different environments and operates under different conditions for which it is impossible or at least impractical to measure or even estimate the thermal load. Thus, the conventional control methods consider the thermal load as an unmeasured disturbance that is rejected by a feedback controller.
Conventional feedback controllers control the VCS reactively, i.e., by reacting to a change in a state of the VCS. In contrast, a forward control method controls the system predictively, i.e., predicting the state of the controlled system and generating the control values based on that prediction. However, the formulation of the forward control methods requires an accurate prediction of all disturbances on the controlled system. Because the thermal load cannot be predicted or accurately estimated, the application of the forward control methods to the VCSs is problematic.
Therefore, feedback controllers, such as proportional-integral (PI) type controllers, are currently the most common type of controllers for controlling the operation of the VCSs, see, e.g., EP 2402662 A1 and U.S. Pat. No. 5,077,983. In the PI controllers, each individual control input is used to control a specific system output to a desired value. This means that for a multivariable system, the control designer is forced to specify individual input-output pairings for each PI controller. Such individual controllers can reduce the optimality of the joint operation of various components of the VCS.
During the operation of the VCS, various constraints should be enforced. For example, certain maximum or minimum temperatures and pressures should not be violated for equipment safety. The feedback controllers enforce constraints reactively, i.e., corrective action is taken once a dangerous situation is detected. The violations of the constraints can occur for some period of time while the system responds with corrective actions, and therefore the threshold at which corrective action is used is selected conservatively to account for violations that can occur. As a result, the feedback controllers with reactive constraint management logic are often detuned away from the value of the constraints, which sacrifice the regions of highest performance, see, e.g., EP2469201.
Accordingly, there is a need in the art for a system and a method for an efficient control of vapor compression system subject to constraints.